The present invention relates to a powder coating slurry, especially a powder clearcoat slurry, comprising at least one hydroxyl-containing binder A and a crosslinking agent B containing free isocyanate groups, and also water, the components being in the form of microencapsulated particles. The invention further relates to a process for preparing such a powder coating slurry and to its use for producing coated substrates.
In the context of the present invention, a powder coating slurry is a dispersion of particles of solid film-forming coating materials in water. Coating material particles may be of like or different composition. For example, the particles may comprise both a binder A or a binder mixture A and a crosslinking agent mixture B. Alternatively, a fraction of the particles of coating material may consist substantially of the binder A or binder mixture A, and another fraction may consist substantially of the crosslinking agent B or crosslinking agent mixture B. Furthermore, the particles of coating material may comprise customary and known additives, where these are not present separately in solution or dispersion in the water.
For the production of clearcoats, a very wide variety of coating materials are known. Clearcoats of high resistance to chemicals, solvents, acid rain and weathering are obtained from coating materials based on hydroxyl-containing polyacrylates (polyhydroxy-polyacrylates) which are crosslinked using free polyisocyanates. It is disadvantageous here firstly that the systems in question are two-component systems and secondly that the fraction of organic solvents is in the range from 40 to 45% by weight.
In order to avoid solvent emissions, therefore, a variety of powder clearcoat materials were developed.
For instance, powder clearcoat materials based on hydroxyl-containing polymers and block polyisocyanates are known where the blocking agents used comprise epsilon-caprolactam and/or ketoximes. Disadvantages of these powder coating materials are the need for special application equipment, sensitivity to contamination, the need for high film thicknesses for practical leveling, and high baking temperatures. The requirement for special application equipment is a hindrance to the integration of the powder coating technologies into existing coating lines, for economic and technological reasons, especially since it necessitates a partial refit of the coating lines. The requirement for high film thicknesses results from the relatively large particles, of approximately 25 micrometers in size, and is disruptive on economic grounds and for reasons of the weight.
Also known are powder clearcoats based on epoxy-functional polyacrylates which can be crosslinked with polycarboxylic acids or their anhydrides. These too have the disadvantages set out above. Moreover, they fail to attain the resistance of solventborne two-component clearcoats. The same applies to the known powder clearcoats based on hydroxyl-containing polyacrylates and polyisocyanates in accordance with the German patent DE-A-44 06 157.
Powder coating slurries are known, for example, from patents DE-A-196 13 554 and WO 96/32452. They are based on epoxy-functional polyacrylates which are crosslinked using polycarboxylic acids. These known powder coating slurries can be processed on conventional application equipment and contain virtually no organic solvents. However, they are capable of further optimization, so that the clearcoats produced from them attain the properties profile of the two-component clearcoats.
As far as the term xe2x80x9cfree isocyanate groupsxe2x80x9d is concerned, in the context of the present invention the following explanations and definitions are necessary. A polyisocyanate whose isocyanate groups have predominantly been protected with a blocking agent contains virtually no remaining free isocyanate groups. The use of such block polyisocyanates in coating materials, however, has the disadvantage that, when the isocyanate groups are liberated during baking, there is very considerable release of blocking agent and products resulting therefrom, which is disadvantageous for reasons of environmental protection and which frequently leads to lower popping limits. The latter means that, during baking, gas bubbles are formed in the coatings if a certain film thickness is exceeded. Consequently, polyisocyanates were developed which were present in the form of discrete particles and whose polyisocyanate groups at the particle surface were reacted with a deactivator. The concept of reaction in this case embraces not only chemical reactions of the superficial isocyanate groups with the deactivator but also a physical blocking of the surface of the particles, by physisorption or chemisorption, for example. Within the volume of the particles, the isocyanate groups are in any case free. The proportion of the isocyanate groups reactive with a deactivator in relation to the free isocyanate groups of the volume of a particle is usually below 0.3, as a general rule considerably below 0.1. Consequently, microencapsulated polyisocyanates of this kind, as they are known, are counted among the unblocked polyisocyanates. Where a microencapsulated polyisocyanate is heated with a binder, the polyisocyanate particles melt and the formerly purely geometrically and/or physically protected isocyanate groups of the volume become accessible for the crosslinking reactions and are able to crosslink the binder.
Microencapsulation is known per se and is described, for example, in the patents DE-A-32 28 670, DE-A-32 28 724, DE-A-31 12 054, DE-A-32 30 757, U.S. Pat. No. 4,888,124 or EP-A-0 510 476. Here, in general, the solid, finely ground polyisocyanate is dispersed in a liquid polyol which at the same time forms the reactive countercomponent for crosslinking. These systems are used to produce foams or underbody protection. They are not suitable for the preparation of coating materials where the surface leveling and gloss requirements are high.
It is an object of the present invention to find a one-component coating material, especially a one-component clearcoat material, which in particular produces clearcoats which match the clearcoats produced from two-component coating materials, especially in terms of gloss, and which may be applied using the customary spray application equipment while exhibiting as low a degree of pollutant emissions as powder clearcoat materials. Furthermore, the one-component coating material should have excellent storage stability and good leveling, even at a low film thickness.
The invention accordingly provides the novel powder coating slurry, in particular the novel powder clearcoat slurry, comprising at least one hydroxyl-containing binder A, at least one polyisocyanate crosslinking agent B, and water, where
1) the hydroxyl-containing binders A and the polyisocyanates B have been homogenized such that not less than 30, preferably not less than 60 and in particular not less than 90% by weight of the polyisocyanate B are present in solution or dispersion in the particles comprising the binders A, and
2) the particles of the polyisocyanate B which may still be present in the aqueous phase, and the particles comprising the binders A and the polyisocyanates B, have been stabilized, by way of the isocyanate groups present on their surface, by means of a deactivator added to the aqueous phase.
Surprisingly, therefore, unblocked polyisocyanates B may be dispersed together with a binder A in water so as to form a storage-stable powder coated slurry if the microencapsulation of the particles in the aqueous phase is conducted by means of the deactivator. Since the deactivation is predominantly not carried out with the binder A, it is possible to obtain coatings, especially clearcoats, having very good leveling and high gloss, even at film thicknesses which are atypically low for powder coatings. The coatings and clearcoats produced from the powder coating slurry of the invention have resistance properties which come close to those of clearcoats produced from two-component systems. Finally, the operation is virtually emission-free, since organic solvents are present only in very small amounts, if at all, in the powder coating slurry of the invention.
For the powder coating slurry of the invention it is important that the homogenization proceeds to such an extent that not less than 30, preferably not less than 60, and in particular not less than 90% by weight of the polyisocyanate B are present in distribution, i.e., dissolved or dispersed, in the discrete binder particles A.
In accordance with the invention, the polyisocyanates B are present together with the binders A in the particles. For practical reasons, however, it is possible for less than 100% by weight of the polyisocyanates B to be present alongside the binders A in the particles. In this case the polyisocyanates B form particles which are present as a separate, discrete phase. It is important that not less than 30, preferably not less than 60 and in particular not less than 90% by weight of the polyisocyanates B are present in the particles in which the binders A are present. In other words, not more than 70, preferably not more than 40 and in particular not more than 10% of the polyisocyanates B should be present as separate discrete particles.
In accordance with the invention, it is of advantage if the binders A and crosslinking agents B are used in a proportion such that the molar ratio of hydroxyl groups to isocyanate groups is from 0.6:1 to 1:1.4, in particular from 0.8:1 to 1:1.2.
Furthermore, customary additives may be present, especially devolatilizers, leveling agents, light stabilizers such as UV absorbers and free-radical scavengers, defoamers, antioxidants, and stabilizers, the additives being present in either the particles and/or the aqueous phase. Moreover, at least one catalyst may be present for the reaction of the hydroxyl groups with the isocyanate groups.